1. Field of the Invention
The present invention relates to a technique of effecting an exposure of a sample to a charged particle beam. More particularly, it relates to a constitution of an apparatus and a method for forming a fine pattern on a sample (wafer) using an electron beam.
Recently, with a high density of integrated circuits, photolithography which has heretofore been a primary technique of forming a fine pattern has been substituted by a novel technique of effecting an exposure using electrons (electron beam), and the novel technique has practically come into use.
2. Description of the Related Art
A conventional electron beam exposing apparatus is a pattern-drawing apparatus which deflects and scans an electron beam having a variable rectangular section on a sample (wafer) to form a pattern. Namely, it is an apparatus having a function of generating a "hardware" pattern from a "software" pattern data.
Since a prior art exposure has been realized by successively linking a one-shot beam having a rectangular cross section to draw a pattern, a drawback occurs in that, the smaller the pattern size becomes, the more the exposing shot per unit size (unit square) is increased in number. This causes a lowering in throughput. To cope with this, a so-called block pattern transfer type exposing technique has been proposed which is intended to obtain a practical throughput even in the forming of an ultra-fine pattern.
In a semiconductor device necessitating the drawing of an ultra-fine pattern, e.g., a dynamic random access memory (DRAM) having a capacity of nominal 64M (1M=10.sup.6), there is often seen a case that a greater part of the area, which is the object of exposure although it is a fine pattern, is constituted by a repetition of a certain basic pattern. In this case, where the basic pattern can be generated with a single shot (one shot) irrespective of a complexity thereof, it becomes possible to effect the exposing processing with a constant throughput irrespective of a fineness of the pattern per se. In this regard, there is proposed the above block pattern transfer type exposure which applies an electron beam to a transmission mask on which the basic pattern is formed, generates one-shot beam having a section of the shape of the basic pattern, and successively links the one-shot beam to form a repetition pattern.
In an example of the prior art block pattern transfer type exposure, a transmission mask having a basic block pattern in the form of an opening formed into a predetermined shape is employed, as described later in detail. The exposing processing is carried out by transmitting an electron beam through the opening, giving optical treatments by means of lenses, deflectors, or the like, to the electron beam having a section of the predetermined shape, and then projecting the shaped electron beam on a wafer.
In this case, a reduced transfer image of the opening is projected on the wafer. Accordingly, where the transmission mask has a plurality of different block patterns, a disadvantage occurs in that, under a condition that the shaped beam is not deflected back to an original optical axis, a position of a transfer image corresponding to each of the block patterns is different from each other depending on geometrical conditions (arrangement and shape) of a selected block pattern. This is an obstacle to the forming of a repetition pattern by link of transfer images (one-shot beams).
To cope with this, there is proposed a technique in which quantity of deflection of the shaped electron beam is suitably controlled in a downstream side of the transmission mask and, even if any one of the block patterns is selected, a transfer image corresponding to the selected block pattern is controlled to be projected at a substantially identical position on the wafer. Even by use of the technique, however, it is difficult to precisely bring the respective position of transfer images corresponding to all of the block patterns to an identical position on the wafer. Problems in the prior art will be explained in detail later in conjunction with the preferred embodiments of the present invention.